The major difference between catalysts known in the art for converting o-xylene and naphthalene to phthalic anhydride has been in the catalyst support that gives the best results; conversion of o-xylene has been thought to require the use of a titania bearing support with titania preferably in the anatase form, whereas straight silica is the preferred support for naphthalene oxidation. Fluidized beds using naphthalene feed are generally operated at throughputs (ratio of hydrocarbon feed per hour to the weight of catalyst, F/W) of about 0.05, and alkali metal sulfates are usually included in the catalysts used in these reactors to moderate the reaction, reducing the activity of the catalyst but increasing the selectivity to phthalic anhydride. Such catalysts favor cleavage of the aromatic ring followed by oxidation of the exposed carbons.
Fixed-bed reactors using o-xylene feed are operated at high F/W ratios such as 0.20 and can utilize a much more active catalyst. The vanadia-titania catalysts employed favor oxidation of the methyl groups present rather than cleavage of the aromatic ring. The differences among the various commercial catalyst compositions are slight; some employ variations in the promoters such as potassium oxide, antimony oxide and phosphorous oxides, and some are used in two stages, with the reaction mixture initially contacting a catalyst of relatively low activity and then a catalyst of high activity.
A fluid-bed catalyst and process disclosed in U.S. Pat. No. 3,565,919 to Friedrichsen et al was said to give satisfactory yields of phthalic anhydride using orthoxylene feed. However, titania-silica produced by the teachings of U.S. Pat. No. 3,565,919 results in a substantial portion of the titania being present in the rutile phase. Saleh, Wachs, Chan and Chersich, "Comparison of V.sub.2 O.sub.5 /TiO.sub.2 (Anatase) and V.sub.2 O.sub.5 /TiO.sub.2 (Rutile):Promoting Effect of the Support", Symposium on Fundamental Chemistry of Promoters and Poisons in Heterogeneous Catalysis Presented Before the Division of Petroleum Chemistry, Inc., American Chemical Society, New York Meeting, Apr. 13-18, 1986, pages 272-276; and Gasior, Gasior and Grzybowska "O-Xylene Oxidation on the V.sub.2 O.sub.5 -TiO.sub.2 Oxide System, Dependence of Catalytic Properties on the Modification of TiO.sub.2 ", Applied Catalysis, 10 (1984) pages 87-100 using orthoxylene oxidation catalysts produced with anatase and with rutile titania, shows that the former effect more selective oxidation to phthalic anhydride, while the PA and C-8 yields with catalysts containing rutile were much lower. The literature (Sembaev, Suvorov, Saurambaeva and Ivanovskaya, "Principles for the Selection of Vanadium Oxide Catalysts for Partial Oxidation and Oxidative Ammonolysis of Aromatic Compounds", Geterog. Katal., Mater. Vses Konf. Mekh. Katal, Reakts., 3rd, 1981, [published 1982] pp. 58-61) further shows that rutile TiO.sub.2 is isomorphous with V.sub.2 O.sub.5 which suggests that these oxides will form solid solutions during prolonged exposure at high (reaction) temperatures thereby limiting catalyst life. The form of TiO.sub.2 present in the catalyst is not addressed in U.S. Pat. No. 3,565,919.
A catalyst disclosed in U.S. Pat. No. 3,862,960 to Cheavens et al containing a relatively high percentage of alkali pyrosulfates reported good yields at air-to-feed weight ratios of 10 and F/W of 0.05, which are appropriate rates for fluid bed operation; however, titania was employed as the catalyst support. Titania does not fluidize well and does not have enough attrition resistance to be useful for a practical length of time in a fluidized bed.
One of the catalysts and one of the processes representative of the prior art are described in UK Pat. Application GB No. 2104066 A. FIG. 1 of that patent confirms that the yields obtained using naphthalene as a feedstock are different from those obtained with o-xylene over the same catalyst, and that, with the catalysts of the prior art, the yields obtained from mixtures of the two progress linearly from the yield obtained with 100 percent of one hydrocarbon to the yield obtained with 100 percent of the other.
It is an object of our invention to provide a versatile catalyst composition that can be used in either a fixed- or fluidized-bed process for the catalytic oxidation of o-xylene and or naphthalene to phthalic anhydride, one that can accommodate the inclusion of sulfur-containing promoter with the reactants, one that will enable processing of impure naphthalene, and one that can be used to improve phthalic anhydride-manufacturing processes of the prior art. It is another object of our invention to provide a catalyst and process for the manufacture of phthalic anhydride which can convert mixtures of o-xylene and phthalic anhydride, providing higher yields and therefore more economical manufacture than any in the prior art.